Honor deflateReset promise to reset the header.

This corrects the code to match the documentation in zlib.h.
However, note that this may break code out there that assumed the
incorrect persistence of the header across resets.

deflate.c

diff --git a/deflate.c b/deflate.c
index a5be5e6..59220e4 100644
--- a/deflate.c
+++ b/deflate.c
@@ -445,7 +445,6 @@
     s->status = INIT_STATE;     /* to pass state test in deflateReset() */
 
     s->wrap = wrap;
-    s->gzhead = Z_NULL;
     s->w_bits = (uInt)windowBits;
     s->w_size = 1 << s->w_bits;
     s->w_mask = s->w_size - 1;
@@ -671,6 +670,8 @@
         adler32(0L, Z_NULL, 0);
     s->last_flush = -2;
 
+    s->gzhead = Z_NULL;
+
     _tr_init(s);
 
     return Z_OK;